1. Field
The present disclosure generally relates to motor control and, more particularly, relates to resolvers operable to detect motor speed.
2. Description of the Related Art
The permanent magnet (PM) synchronous motor possesses many appealing characteristics for various applications, including pure-electric or hybrid-electric vehicles. Interior permanent magnet (IPM) synchronous motors are used in some electric vehicle traction drives due to their positive features, such as high efficiency and high power density. Such applications require the motor drives to work in a wide speed range while maintaining high efficiency.
Control systems are used to accurately control operation of the electric motors. One input to such control systems is rotor angular position information. Rotor angle may be detected by a resolver. A resolver is an electromechanical device that detects rotor angle and outputs information corresponding to the detected rotor angle. Some types of resolvers are single or multiple pole-pair machines. Exemplary machine-type resolvers may be physically coupled to the rotor shaft or may be coupled to the rotor shaft with a gear or the like.
Output of a resolver is an electrical signal that corresponds to rotor angle. One exemplary type of resolver outputs a sine wave and a cosine wave that are provided to a resolver-to-digital (R/D) converter, which converts the sine/cosine analog output of the resolver into a digital signal corresponding to the rotor's absolute angular position. The digital signal may be used as an input to the control system that controls operation of the motor.
Machine-type resolvers may be single pole-pair or multiple pole-pair machines. A single pole-pair resolver outputs a signal that is directly proportional to the angular position of the rotor. For example, the output of a single pole-pair resolver may correspond to the rotor's mechanical angle from zero degrees (0°) to 360°, or 2π radians. A single pole-pair resolver would output a signal corresponding to 0° to 360° of electrical angle, where the output electrical angle corresponds to the angular position of the rotor. With a two pole-pair resolver, one mechanical revolution of the rotor (0° to 360°) would cause the resolver to output a signal corresponding to 0° to 720° of electrical angle. Electrical output of multiple pole-pair resolvers may be generally represented as the number of pole-pairs (n) times 360° (n×360°) or the number of pole-pairs times 2π radians (n×2π) for one revolution of the rotor. Speed and/or acceleration of the rotor, and hence the motor, may be determined by observing rotor angle change with respect to time.
Various sources of angle error in the output signal of the resolver occur. Such error may arise from mechanical and/or electrical characteristics of the resolver and/or rotor. FIG. 1 is a graph 100 illustrating an exemplary resolver output angle error curve 102 from a two pole-pair resolver (not shown).
Four observations may be made regarding the output angle error of the exemplary two pole-pair resolver. First, in the ideal case of an accurate resolver, the output angle error curve would typically be on the zero axis. The relative distance of the output angle error curve 102 from the zero axis, at any given point, indicates an amount of resolver output angle error at the corresponding angular position of the rotor.
Second, the envelope of first half 104 of the output angle error curve 102 typically does not match the envelope of the second half 106 of the output angle error curve 102. That is, error in the output signal of the resolver generated by the first pole-pair and the second pole-pair, at any given point on the output angle error curve 102, are different.
Third, the period of the first half 104 of the output angle error curve 102 typically does not match the period of the second half 106 of the output angle error curve 102. That is, the length of the first half 104 and the second half 106 of the output angle error curve 102 are different.
Fourth, noise typically induces additional error in the resolver output signal, which is visible as the “width” of the output angle error curve 102. The width of the illustrated output angle error curve 102 corresponds to noise. Noise induced error is not constant over a single rotation of the rotor. In the ideal case of a noiseless resolver, the width of the output angle error curve 102 would be relatively narrow.
The above-described errors in the output angle error curve 102 may arise from a variety of reasons. For example, in the machine-type two pole-pair resolver case, a slight misalignment between the rotor and the stator may result in periodic nonlinear angle measurement errors and differences between the length of the first half 104 and the second half 106 of the output angle error curve 102. Differences in winding turns, winding lengths, and winding orientation of the winding around the pole-pairs of a machine-type resolver will cause errors in the output angle error curve 102. Variations in the physical shape and/or electrical characteristics of the rotor may also appear as errors in the output angle error curve 102. Electrical circuits associated with the resolver, which may be out of tune and/or miscalibrated, may also cause errors in the output angle error curve 102. Other sources of angle error are known, and are not described in detail herein for brevity.
Existing motor control methods and apparatus, particularly in PM or IPM motors used in electric or hybrid vehicle applications, may perform poorly when the resolver does not provide accurate rotor angular position information. That is, motor torque or vehicle torque may be difficult to control and therefore, speed and acceleration of the vehicle may, in turn, be difficult to control if accurate rotor angle cannot be accurately and/or reliably determined because of resolver error. Accordingly, a more accurate resolver method and apparatus is desirable.